1. Field of the Invention
The present invention relates to a process for fabricating semiconductor device having a multilayer wiring structure. More particularly, the present invention relates to a technique for flattening a base layer on which the wiring is formed.
2. Description of the Related Art
In recent years, to increase the degree of integration in semiconductor integrated circuits, study for a constitution which increases not only the two-dimensional, but the three-dimensional degree of integration is being studied. A multilayer wiring is necessary to increase the three-dimensional degree of integration.
However, with an increase in the number of superposed layer of wiring, inevitably increases the surface roughness, and hence problems arise in which, for instance, relaxation of the design rule is required as the formation of layers progress to the upper layer.
As a method for overcoming the problem above, there is a known technique disclosed in Japanese Unexamined Patent Application in which the surface of a multilayer wiring is flattened.
It is still difficult, however, to flatten, as a whole, an interlayer insulating film having surface irregularities and the electrodes and wirings that protrude from the contact hole formed on the interlayer insulating film.
For instance, when a multilayer structure as shown in FIG. 3 comprising two layers of electrodes or wirings is formed, the surface thereof includes the irregularities of the interlayer insulating film 301 and the irregularities caused by the wirings or electrodes 302 to 304 having contacts with the lower wiring.
Such irregularities become obstacles in case a multilayer wiring is to be formed further thereon. However, it is difficult to wholly flatten such irregularities.
Further, from the problem of covering the inside of the contact holes for the electrodes 302 to 304, disconnections and contact failure occur on the side planes inside the contact hole 300.
To overcome the above problem, it is necessary to form the electrode material thick so enough as to fill the inside of the contact hole with a material constituting the electrode. However, if such a measure is taken, the electrode itself becomes a large protrusion, and brings about a novel problem ascribed thereto.